The measurement of the concentration of a vapor in a gas is often important For example, it is useful to know the concentration of flammable gases in a gas stream in combustion technology. The humidity of air in an area is of interest to people concerned about the weather. Further, in health care, an anesthesiologist would want to know the concentration of an anesthetic in a gas stream administered to a patient during surgery. The concentration of water vapor in the exhaled air of a person can indicate the functioning condition of the person's respiratory system. The detection of temperature and moisture content of air being inhaled and exhaled will provide valuable information to health care professionals on aerosol therapy and toxicology of toxic gases inhalation.
Vapor concentration sensors based on measuring the mass of vapor absorbed on polymer films coated on surface acoustic wave devices have been developed. For example, Jay W. Grate and Mark Kluxty, Anal. Chem., vol. 63, pp 1719-1727 (1991), describe a humidity sensor in which vapor absorption changes the frequency of oscillation of mass-sensitive resonators. Also, Polymer-based impedance effect humidity sensors are disclosed by S. Tsuchitani et al. in "A humidity sensor using ionic copolymer and its application to a humidity--temperature sensor module," Sensors and Actuators, Vol. 15, No. 4, pp 375-386, 1988. In the Tsuchitani humidity sensors, moisture absorption by ionic copolymers causes a change in impedance in an electrical circuit, thereby causing a change in oscillation frequency. However, vapor concentration sensors by vapor absorption are not very specific and are subject to interference by any absorbable vapor that has not be present in a calibration sample. Moreover, such vapor sensors do not work well near the condensation point because they may not respond to a fall in humidity quickly. Therefore, a need exists for a highly specific vapor concentration sensor that will function over a wide range of concentrations.
Humidity sensors have been used for many years to determine air humidity for weather reporting. For such applications, one simple kind of humidity sensor has a dry bulb thermometer and a wet bulb thermometer. The wet bulb thermometer has a thermometer with a bulb moistened by a wick. Generally water passes by capillary action against gravity up the wick from a container. Water evaporates from the wick when the air is unsaturated with water vapor. Due to the cooling effect of the evaporation of water from the wick, the temperature of the wet thermometer will be lower than the true temperature of the air had there been no evaporation. The temperature of the wet thermometer is known as the "wet-bulb temperature." The temperature that is measured by a dry thermometer, known as the "dry-bulb temperature," and the wet-bulb temperature are used to determine the humidity in air. See, for example, McCabe and Smith, Unit Operations of Chemical Engineering, McGraw-Hill, Ch. 24, 3rd ed., (1956). Such humidity sensors tend to be large and requires a generally upright position to keep the liquid water in place. Their response time is typically not very fast.
More recently, moisture sensors employing micro-thermocouple sensors for determining temperature and relative humidity in an airstream have been reported, for example, in "Design and development of a micro-thermocouple sensor for determining temperature and relative humidity patterns within an airstream," J Biomechan. Eng. Vol. 111, PP. 283-287, Nov. 1989. In such a device, a wet-bulb thermocouple junction is coated with a sprayed-on boron nitride coating, which is reported to be hard and porous. A sleeve is used to supply water to the boron nitride coating. It would appear that coating a thermocouple junction by spraying is not an easy task and one has to take special care to position the sleeve precisely to wet the boron nitride coating without leakage. It is also difficult to form a boron nitride coating that is stable on metal or glass surfaces. Moreover, to get a porous structure suitable for conducting water adequately one needs to form a boron nitride layer that is quite thick, making it brittle and slow to transfer heat.
Therefore, a need exists for a dry-bulb-wet-bulb vapor concentration sensor that is relatively simple to construct, and particularly for a dry-bulb-wet-bulb vapor concentration sensor that is sturdy.